A significant role in the development of integrated service digital network (ISDN) has been played by the 144 kbit/s transmission technique used for the access to the network by the subscriber. This technique serves to ensure a low error rate and low spectral content of the signal at high frequencies to avoid inconvenience due to crosstalk, is compatible with HiFi diffusive services and allows integration of the entire system so as to reduce cost and energy or manpower consumption and to increase reliability.
One of the main problems encountered in echo-cancellation transmission techniques is the necessity to ensure a useful-signal/residual-echo or signal-to-noise ratio (S/N) of at least 20-25 dB, so as to allow correct reception.
This S/N ratio is particularly difficult to obtain when a system is to operate also with a 40 dB line attenuation. In fact, hybrid matching to the line, seldom exceeds 10 dB; that is why an echo suppression of at least 50-55 dB is necessary to reach the desired S/N ratio value.
Echo estimation and cancellation are generally performed by a transverse-circuit or memory-type structure, which can ensure the desired accuracy to the samples on which they operate.
Memory structures can recover even nonlinear distortions present in transmission and reception circuits, while the transverse-circuit types are profitably used to cancel echo portions which do not effect these types of distortions, as they require simpler circuit designs.
However, to attain overall cancellation values of 50-55 dB, it is necessary to estimate and cancel the echo over a very high number of samples, since the queues extend over very long time intervals. The use of transverse circuit or memory cancellers for this purpose is extremely burdensome. In fact, it is necessary to operate on a very high number of samples, e.g. 25 to 35. This entails, besides increased circuit complexity, also considerably high processing speed, and prevents system implementation in CMOS technology.
The latter point is most important because CMOS technology is, nowadays, the only developed technology capable of meeting a significant number of requirements such as low-power consumption and high integration density, which can allow ISDN development.